This is xnu-8019. See this file in:
/*
 * Copyright (c) 2016 Apple Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_LICENSE_HEADER_END@
 */
#include <mach/mach_time.h>

extern kern_return_t
mach_timebase_info_trap(mach_timebase_info_t info);

kern_return_t
mach_timebase_info(mach_timebase_info_t info)
{
	static mach_timebase_info_data_t cached_info;

	/*
	 * This is racy, but because it is safe to initialize twice we avoid a
	 * barrier in the fast path by risking double initialization.
	 */
	if (cached_info.numer == 0 || cached_info.denom == 0) {
		kern_return_t kr = mach_timebase_info_trap(&cached_info);
		if (kr != KERN_SUCCESS) {
			return kr;
		}
	}

	info->numer = cached_info.numer;
	info->denom = cached_info.denom;

	return KERN_SUCCESS;
}